I. Introduction.
This invention relates to electroplating nonconductors, and more particularly, to a process for electroplating the surface of a nonconductor by converting an adsorbed colloid into a chemically resistant, metal chalcogenide conversion coating, which functions as a base for direct electroplating. The invention is particularly useful in the manufacture of printed circuit boards where copper is deposited over a circuit board base material.
II. Description of the Prior Art.
Nonconductive surfaces are conventionally metalized by a sequence of steps comprising catalysis of the surface of the nonconductor to render the same catalytic to an electroless metal deposit followed by contact of the catalyzed surface with an electroless plating solution that deposits metal over the catalyzed surface in the absence of an external source of electricity. Metal plating continues for a time sufficient to form a metal deposit of the desired thickness. Following electroless metal deposition, the electroless metal deposit is optionally enhanced by electrodeposition of a metal over the electroless metal coating to a desired full thickness. Electrolytic deposition is possible because the electroless metal deposit serves as a conductive coating that permits electroplating.
Catalyst compositions used for electroless metal plating are known in the art and disclosed in numerous publications including U.S. Pat. No. 3,011,920, incorporated herein by reference. The catalysts of this patent consist of an aqueous suspension of a tin noble or precious (catalytic) metal colloid. Surfaces treated with such catalysts promote the generation of electrolessly formed metal deposits by the oxidation of a reducing agent in an electroless plating solution catalyzed by the catalytic colloid.
Electroless plating solutions are aqueous solutions containing dissolved metal and reducing agent in solution. The presence of dissolved metal and reducing agent together in solution result in plate out of metal in contact with a catalytic metal tin catalyst. However, the presence of the dissolved metal and reducing agent together in solution can also result in solution instability and indiscriminate deposition of metal on the walls of containers for such plating solutions. This may necessitate interruption of the plating operation, removal of the plating solution from the tank and cleaning of tank walls and bottoms by means of an etching operation. Indiscriminate deposition may be avoided by careful control of the plating solution during use and by the use of stabilizers which inhibit indiscriminate deposition, but which also retard plating rate.
Attempts have been made in the past to avoid the need for an electroless plating solution by a direct plating process whereby a metal may be deposited directly over a treated nonconductive surface. One such process is disclosed in U.S. Pat. No. 3,099,608, incorporated herein by reference. The process disclosed in this patent involves treatment of the nonconductive surface with a tin palladium colloid which forms an essentially nonconductive film of colloidal palladium particles over the nonconductive surface. This is the same tin palladium colloid used as a plating catalyst for electroless metal deposition. For reasons not fully understood, it is possible to electroplate directly over the catalyzed surface of the nonconductor from an electroplating solution though deposition occurs by propagation from a conductive surface. Therefore, deposition begins at the interface of a conductive surface and the catalyzed nonconductive surface. The deposit grows epitaxially along the catalyzed surface from this interface. For this reason, metal deposition onto the substrate using this process is slow. Moreover, deposit thickness is uneven with the thickest deposit occurring at the interface with the conductive surface and the thinnest deposit occurring at a point most remote from said interface.
An improvement in the process of U.S. Pat. No. 3,099,608 is disclosed in U.K. Patent No. 2,123,036B, incorporated herein by reference. In accordance with the process described in this patent, a surface is provided with metallic sites and the surface is then electroplated from an electroplating solution containing an additive that is said to inhibit deposition of metal on the metal surface formed by plating without inhibiting deposition on the metallic sites over the nonconductive surface. In this way, there is said to be preferential deposition over the metallic sites with a concomitant increase in the overall plating rate. In accordance with the patent, the metallic sites are preferably formed in the same manner as in the aforesaid U.S. Pat. No. 3,099,608--i.e., by immersion of the nonconductive surface in a solution of a tin palladium colloid. The additive in the electroplating solution responsible for inhibiting deposition is described as one selected from the group of dyes, surfactants, chelating agents, brighteners and leveling agents. Many of such materials are conventional additives for electroplating solutions.
There are limitations to the above process. Both the processes of the U.S. and the U.K. patents for direct electroplating require conductive surfaces for initiation and propagation of the electroplated metal deposit. For this reason, the processes are limited in their application to metal plating of nonconductive substrates in areas in close proximity to a conductive surface.
One commercial application of the process of the U.K. patent is the metallization of the walls of through holes in the manufacture of double-sided printed circuit boards by a process known in the art as panel plating. In this application, the starting material is a printed circuit board substrate clad on both of its surfaces with copper. Holes are drilled through the printed circuit substrate at desired locations. For conductivity, the hole walls are catalyzed with a tin palladium colloid to form the required metallic sites on the surfaces of the walls of the through holes. Since the circuit board material is clad on both of its surfaces with copper and the circuit board base material is of limited thickness, the copper cladding on the surfaces of the circuit board material is separated by the thin cross section of the substrate material. The next step in the process is direct electroplating over the catalyzed hole walls. Since the copper cladding on each surface is separated by the cross section of the substrate, during electroplating, deposition initiates at the interfaces of the copper cladding and the through hole walls and rapidly propagates into the holes. The hole wall is plated to desired thickness within a reasonable period of time. Thereafter, the circuit board is finished by imaging and etching operations.
A disadvantage to the above panel plating process is that copper is electroplated over the hole wall and over the entire surface of the copper cladding. The steps following plating involve imaging with an organic coating to form a circuit pattern and removal of copper by etching. Therefore, copper is first electrolytically deposited and then removed by etching, a sequence of steps which is wasteful of plating metal, etchant and time, and therefore, more expensive.
The art, recognizing the disadvantages of panel plating, has developed a method for manufacture of printed circuit boards known as pattern plating. In this process, a printed circuit board base material is drilled at desired locations to form through holes. The through holes are metalized using conventional electroless plating techniques. Electroless copper is plated onto the walls of the through holes and over the copper cladding. Thereafter, photoresist is applied and imaged to form the circuit pattern. The board is then electroplated with copper depositing on the copper conductors and through hole walls, but not over the entire surface of the copper cladding. Solder mask is then plated over the exposed copper by immersion or electroplating and the remaining photoresist is stripped. The copper not protected by the solder is then removed by etching to form the copper circuit.
Pattern plating cannot be used with the metalizing process of the aforesaid U.K. patent. The treatment of the copper cladding prior to the application of the photoresist and the development of the photoresist, all as required for pattern plating, requires the use of treatment chemicals found to dissolve or desorb the tin palladium colloid from hole walls. Since this occurs prior to electroplating, direct electroplating to provide conductive through holes becomes impossible.
Copending U.S. Pat. application Ser. No. 07/071,865, filed July 10, 1987 and assigned to the same assignee as the subject application, provides a new method for direct electroplating of the surface of a nonconductor and to articles manufactured by said method. The process is in an improvement over the process of the above referenced U.K. Patent.
The invention disclosed in the copending application was predicated upon a combination of discoveries. One discovery was that sulfide films of metals that function as electroless deposition catalysts may be electroplated directly without requiring an intermediate electroless coating. Another discovery of the invention was that many of such sulfide films are insoluble and unaffected by treatment chemicals used for plating of plastics and circuit board fabrication and therefore, the process of the invention was suitable for the formation of printed circuits using pattern plating procedures.
The process of the copending patent application is illustrated by the plating sequence that follows and is compared to a conventional plating process requiring electroless metal deposition.
______________________________________ Conventional Process (A) Inventive Process (B) ______________________________________ Step 1 Desmear with chromic or Desmear with chromic or sulfuric acid or plasma sulfuric acid or plasma Step 2 Clean and condition with Clean and condition with detergent type material detergent type material Step 3A Microetch copper cladding -- Step 4 Catalyst predip Catalyst predip Step 5 Catalyze with Catalyze with catalytic colloid catalytic colloid Step 6 Accelerate Accelerate (optional) Step 7 Deposit electroless Treat with sulfide metal solution Step 7B -- Microetch copper cladding Step 8 Electroplate Electroplate ______________________________________
A comparison of the two processes illustrated above demonstrates that the process disclosed in the copending application replaces the need for electroless plating with a direct electroplating step thereby eliminating the need for a costly electroless metal plating solution that may be subject to stability and disposal problems. The elimination of the electroless plating step is accomplished without an increase in the total number of steps required for metal deposition. Further, the process of the invention was found to be unaffected by conventional processing chemicals used for metal plating of plastics and formation of printed circuit boards.
In the process of the copending application illustrated above, contact of the catalytic metal on the surface of the nonconductor with a sulfide treatment solution (Step 7) results in the formation of a metal sulfide conversion coating of the catalytic metal (the catalytic metal sulfide). The sulfide solution may be a simple aqueous solution of a water soluble alkali or alkaline earth metal sulfide or a solution of a covalently bonded sulfide such as a thiocarbonate or a dithiodiglycolate. In accordance with the invention of the copending application, the catalytic metal sulfide formed by treatment with the sulfide solution is a suitable conversion coating for direct electroplating.
For the formation of printed circuit boards using the process of the copending application, it is preferred that an etching step be used subsequent to formation of the catalytic metal sulfide film over the surface of the nonconductor (Step 7B above). This etching step may use the same etchants as used in the conventional process to clean copper cladding (Step 3A above). It is preferred that the etching step be deferred to a point subsequent to the step of formation of the catalytic metal sulfide conversion coating so that the etchant may remove sulfide residues on the surface of the copper cladding. It is an advantage of the process that the catalytic metal sulfide conversion coating over the nonconductive surface is essentially unaffected by the step of etching the copper cladding. It is a further advantage that any residues deleterious to copper-copper bonding left on the copper by a photoresist used in the manufacture of printed circuit boards may be readily removed by a more aggressive etchant than was possible in a conventional plating line where the electroless copper is only about 100 microinches thick over the hole wall.
The final step in the process of the copending application comprised electroplating of the thin catalytic metal sulfide conversion coating. This was accomplished using standard electroplating procedures. The procedures of the above referenced U.K. Patent are suitable for electroplating the sulfide coating described therein.